vint: optimise deserialisation routine
vint: optimise deserialisation routine

At the moment, vint deserialisation is using a naive approach, reading
each byte separately. In practice, vints are going to most often appears
inside larger buffers. That means we can read 8-bytes at a time end then
figure out unneded parts and mask them out. This way we avoid a loop and
do less memory loads which are much more expensive than arithmetic
operations (even if they hit the cache).

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